(a) Field of the Invention
The present invention relates to a method and apparatus for manufacturing liquid crystal displays.
(b) Description of the Related Art
A liquid crystal display (LCD) includes two substrates and a liquid crystal layer interposed therebetween. The transmittance of light is controlled by the strength of the electric field applied to the liquid crystal layer. To manufacture the liquid crystal display, two substrates having electrodes on their inner surfaces, respectively, are assembled to form an empty liquid crystal cell, and then liquid crystal material is injected into the empty cell.
A vertically aligned twisted nematic (VATN) liquid crystal display uses homeotropically aligned liquid crystal having negative dielectric anisotropy. A VATN liquid crystal display has many advantages such as a high contrast ratio, fast response time and wide viewing angle, etc. However, there are some problems in the manufacturing method of VATN LCDs. For instance, as vertical alignment films are used for homeotropic alignment, it takes five times longer than the conventional twisted nematic LCDs to fill the empty cell with liquid crystal material due to the steric hindrance and the interaction between the liquid crystal material and the vertical alignment films.
FIGS. 1 and 2 are sectional view of a conventional twisted nematic liquid crystal display and a VATN liquid crystal display, respectively. The figures are used to describe the filling process for these two conventional LCDs.
As shown in FIG. 1, two transparent insulating substrates 1 and 2 are sealed with a sealant 3, except for an injection hole 6, to define a liquid crystal cell. Alignment films 4 are formed on the inner surfaces of the substrates 1 and 2 to control the alignment of liquid crystal molecules. In the twisted nematic liquid crystal display, homogeneous alignment films are used. The liquid crystal material 5 injected through the injection hole 6 has positive dielectric anisotropy and lies parallel to the substrates 1 and 2 such that the liquid crystal material 5 easily injected into the liquid crystal cell. The arrow indicates the direction of injection.
In the VATN LCD as shown in FIG. 2, homeotropic alignment films 7 are used to align liquid crystal molecules 8 perpendicular to the substrates 1 and 2. The liquid crystal material 8 is injected through the injection hole 6 in a state where the liquid crystal molecules are standing perpendicular to the substrates 1 and 2. Therefore, it takes a longer period of time to fill the cell due in the steric hindrance and the interaction between the liquid crystal material 8 and the vertical alignment films 7.
After injecting the liquid crystal material, an end seal process takes place. In the end seal process, the liquid crystal cell is hot pressed to maintain a uniform cell gap, and then the injection hole is sealed. Generally, an ultraviolet ray hardening sealant (UV sealant) is used for the end seal. That is, a UV sealant is applied over the injection hole, after which the UV rays harden the UV sealant.
However, excess liquid crystal material continuously discharges from the liquid crystal cell after the injection due to the pressure difference between the inside and outside of the liquid crystal cell. Therefore, it is necessary for the liquid crystal cell filled with the liquid crystal material to be undisturbed for two or three hours before undertaking the end seal process. However, it is difficult to estimate a suitable amount of time because the viscosity of the liquid crystal material may vary. Especially in the VATN liquid crystal displays, the time to discharge for the excess liquid crystal material is at least twice as long as the filling process because the liquid crystal molecules are perpendicular to the substrates.
If the excess liquid crystal material is not sufficiently discharged before the end seal process, the cell gap between the substrates is not uniformly maintained. Moreover, if the UV sealant is applied while the liquid crystal material is still discharging through the injection hole, bonding strength is decreased. As a result, the sealant may loosen after hardening. In the worst case, the entire mass of fully-hardened sealant may become completely detached from the LCD. In either case, air becomes mixed with the liquid crystal layer or the liquid crystal material is discharged. In addition, time is wasted because the liquid crystal cell should be left to sit after filling for a long period of time.